Under environments in which an acoustic feedback system is formed in a predetermined space where a speaker and a microphone are provided and a loop in which a sound signal picked up by the microphone is emitted from the speaker and the sound signal is again picked up by the microphone is formed, howling is problematic which occurs as a loop gain of a signal of a specific frequency exceeds 1. In order to suppress the howling, a variety of cancellers have been suggested (for example, refer to Patent Document 1).
A howling canceller of Patent Document 1 has a plurality of notch filters and is connected between a microphone and an amplifier provided at a subsequent stage of the microphone. When a frequency component at which the howling occurs is detected, the howling canceller allots the notch filter to the detected frequency, thereby suppressing the howling.